1. Field of the Invention
The present invention relates to a method and apparatus for forming a thin semiconductor film such as a polycrystalline silicon film on a substrate, a method and apparatus for producing a semiconductor device having such a thin semiconductor film formed on a substrate, and an electro-optical device.
2. Description of the Related Art
Conventionally, vapor phase deposition such as plasma-enhanced chemical vapor deposition (CVD), reduced-pressure CVD, and catalytic CVD, solid phase growth, liquid phase growth, and excimer laser annealing are used to deposit a polycrystalline silicon film for forming drain and channel regions of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) such as a MOSTFT (Metal Oxide Semiconductor Thin Film Transistor).
As disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-131030, Japanese Unexamined Patent Application Publication No. 9-116156, and Japanese Examined Patent Application Publication No. 7-118441, the carrier mobility of an amorphous or microcrystalline silicon film formed by means of plasma-enhanced CVD or reduced-pressure CVD process can be improved by converting the film into a polycrystalline form by performing high-temperature annealing or excimer laser annealing (ELA). The highest carrier mobility achieved by this technique is about 80 to 120 cm2/V·sec.
Because the MOSTFT produced using the polycrystalline silicon film formed by performing ELA on an amorphous silicon film deposited by means of plasma CVD has a rather high electron mobility such as 100 cm2/V·sec, and the MOSTFT can be formed so as to be adapted to high-precision applications. An LCD (Liquid Crystal Display) using MOSTFTs formed of polycrystalline silicon and having a driver circuit integrated on the LCD has attracted considerable attention (Japanese Unexamined Patent Application Publication No. 6-242433). In the excimer laser annealing technique, a film in a precursor form is irradiated with a short-wavelength short-pulse laser beam such as XeCl excimer laser thereby melting and recrystallizing the film in a short time. In this technique, illumination of the amorphous silicon film with the laser beam allows conversion into a polycrystalline form without causing a glass substrate to be damaged. Another advantage of this technique is high throughput.
However, if a MOSTFT is produced using this ELA technique, recrystallization occurs as rapidly as on the order of nsec during the excimer laser annealing process, and thus the grain size of the polycrystalline silicon film formed by the excimer laser annealing process is at most about 100 nm. Even if the substrate is heated to about 400° C. during the irradiation with a short-wavelength short-pulse laser beam to remove hydrogen and oxygen that can inhibit crystal growth and if the solidification speed is controlled, it is difficult to obtain a grain size greater than 500 nm. One known technique to avoid the above problem is to perform laser irradiation repeatedly, for example, 5 times or 30 times, so as to apply large enough energy to obtain a polycrystalline silicon film having a large grain size. However, this technique has other various problems such as instability in the output power of the excimer laser, low productivity, and an increase in cost and reduction in yield/quality that occur when a large-sized apparatus is used. In particular, for a substrate with a large size such as 1 m×1 m, the problems described above become very serious, and it becomes very difficult to achieve high performance/quality at low cost.
In a technique of forming a crystalline silicon film, recently disclosed in for example Japanese Unexamined Patent Application Publication No. 11-97353, an amorphous silicon film is heated at 450° C. to 600° C. for 4 to 12 hours so as to diffuse an element serving as a catalytic element (such as Ni, Fe, or Co) thereby enhancing crystallization of the amorphous silicon film. However, the problem of this technique is that the catalytic element remains in the formed crystalline silicon film. To avoid the above problem, Japanese Unexamined Patent Application Publication No. 8-339960 discloses a technique of removing (gettering) the catalytic element by one of the following methods: performing a heating process in an ambient containing a halogen such as chlorine; performing a heating process after selectively incorporating phosphorus into a crystalline silicon film; and illuminating a crystalline silicon film containing a catalytic element with a laser beam or a high-intensity light ray so as to bring the catalytic element into a state in which the catalytic element can easily diffuse, and then gettering the catalytic element by a selectively added element. However, these methods are complicated, gettering effects are not sufficient, the characteristics of the silicon semiconductor film are degraded, and the stability and reliability of a produced device are degraded.
On the other hand, in the method of producing a polycrystalline silicon MOSTFT by means of solid phase growth, annealing at a temperature higher than 600° C. for 10 hours or longer and formation of a gate oxide SiO2 by means of thermal oxidation at a high temperature of about 1000° C. are required. To perform these processing steps, it is needed to use a semiconductor production apparatus. This limits the substrate size to 8 to 12 inches in diameter. Besides, it is required to use expensive quartz glass to ensure that the substrate can resist high temperatures. This makes it difficult to reduce the cost, and thus applications are limited to EVF, data/AV projector, or the like.
In recent years, a catalytic CVD technique has been developed, which is one of thermal CVD techniques and is capable of depositing polycrystalline silicon film, silicon nitride, or the like on an insulating substrate such as a glass substrate at a low temperature (Japanese Examined Patent Application Publication No. 63-40314, Japanese Examined Patent Application Publication No. 8-250438). This technique is now being improved to put it to practical use. In the catalytic CVD technique, although a carrier mobility of about 30 cm2/V·sec can be obtained without performing annealing for crystallization, the carrier mobility is still not high enough to produce a high-performance MOSTFT. In the case where a polycrystalline silicon film is formed on a glass substrate, an inversion layer (with a thickness of 5 to 10 nm) is formed in the amorphous silicon, depending on the deposition conditions. This makes it difficult to obtain a carrier mobility that is high enough to produce a bottom-gate type MOSTFT. In the LCD using the polycrystalline silicon MOSTFTs and including the integrated driver circuit, the bottom-gate type MOSTFT results in a better production yield and productivity. However, the problem described above results in a bottleneck.